It has been heretofore said that a super-conductive phenomenon appears with a most peculiar nature among various electromagnetical natures exhibited by a certain material. In view of the foregoing nature, it is expected that practical application of the super-conductive phenomenon will largely be widened in near future by utilizing its natures such as complete electrical conductivity, complete antimagnetism, quantification of a magnetic flux or the like.
A high speed switching element, a detecting element having high sensitivity and a magnetic flux measuring instrument having high sensitivity can typically be noted as electronic devices for which the aforementioned super-conductive phenomenon is utilized, and it is expected that these devices are practically used in the wide range of application.
For example, a thin film of Nb.sub.3 Ge deposited on the surface of a substrate by employing a plasma spattering method can be noted as a typical super-conductive material which has been hitherto used for a conventional super-conductive device. However, since the thin film of Nb.sub.3 Ge has a critical superconductivity temperature of about 23.degree. K., the super-conductive device can be used only at a temperature lower than that of a liquid helium. For this reason, when the liquid helium is practically used for the super-conductive device, there arises a significant problem that a cooling cost and a technical burden to be born are increased because of a necessity for installing an equipment and associated instruments for cooling and liquidizing a helium gas with the result that practical use of the super-conductive device not only in the industrial field but also in the household field is obstructed. Another problem is that an absolute quantity of helium source is small and limited.
To obviate the foregoing problems, a variety of endevors have been made to provide a super-conductive material having a higher critical superconductivity temperature. Especially, in recent years, remarkable research works have been conducted for providing a super-conductive thin film composed of an oxide having a higher superconductivity temperature. As a result derived from the research works, a critical superconductivity temperature is elevated to a level of 77.degree. K. This makes it possible to practically operate a super-conductive device having the foregoing super-conductive thin film used therefor while using an inexpensive liquid nitrogen.
To form such a super-conductive thin film composed of an oxide as mentioned above, a spattering method or a vacuum vaporizing/depositing method has been heretofore mainly employed such that the super-conductive thin film is deposited on the surface of a substrate of a single crystal of MgO or SrTiO.sub.3 which is preheated to an elevated temperature.
In addition, with respect to a single crystal employable for the substrate, attention has been paid to a sapphire, YSZ, a silicon, a gallium arsenide, LiNbO.sub.3, GGG, LaGaO.sub.3, LaAlO.sub.3 or the like.
However, it has been found that the conventional method of forming a super-conductive thin film while having a substrate of a single crystal of MgO or a substrate of a single crystal of SrTiO.sub.3 used therefor as a substrate has problems that a critical superconductivity current (Jc) can not stably be elevated, and moreover a critical superconductivity temperature (Tc) is kept unstable.
To form an epitaxial film having excellent properties, it is necessary that a material employable as a substrate satisfactorily meets the following requirements. (I) The lattice constants of substrate crystals are close to that of thin film crystals. (II) A quality of the thin film is not degraded due to mutual diffusion to and from the substrate during an operation for growing an epitaxial thin film. (III) The material employable as a substrate has a melting temperature higher than at lowest 1000.degree. C., since it is heated up to an elevated temperature. (IV) A single crystal having an excellent crystal quality is readily available on the commercial basis. (V) The material employable as a substrate has an excellent property of electrical insulation.
On the other hand, with respect to a super-conductive material composed of an oxide having a higher critical superconductivity temperature, many oxides each in the form of a thin film such as a LnBa.sub.2 Cu.sub.3 O.sub.7-.delta. (.delta.=0 to 1, Ln:Yb, Er, Y, Ho, Gd, Eu or Dy), a Bi-Sr-Ca-Cu-O base oxide, a Tl-Ba-Ca-Cu-O based oxide or the like have been heretofore reported.
All the oxides as mentioned above have lattice constants a and b each of which remains within the range of 3.76 to 3.92 angstroms. When a coordinate system for each oxide is turned by an angle of 45.degree. so that it is visually observed in the turned state, .sqroot.2a and .sqroot.2b are recognized as a basic lattice, respectively. In this case, the lattice constants a and b are expressed such that they remain within the range of 5.32 to 5.54 angstroms.
In contrast with the aforementioned oxides, a magnesium oxide (MgO) which has been widely used as a material for a substrate at present has a lattice constant a of 4.203 angstroms and thereby a differential lattice constant between the aforementioned oxides and the magnesium oxide is enlarged to an extent of 7 to 11%. This makes it very difficult to obtain an epitaxially grown film having excellent properties. In addition, it has been found that things are same with a sapphire, YSZ, a silicon, a gallium arsenide, LiNbO.sub.3 and GGG.
Further, in contrast with MgO, SrTiO.sub.3 has a small differential lattice constant relative to the super-conductive thin film composed of an oxide wherein the lattice constant remains within the range of 0.4 to 4%. Thus, SrTiO.sub.3 is superior to MgO in respect of a lattice matching. However, SrTiO.sub.3 is produced only with a Bernoulli method employed therefor. In addition, SrTiO.sub.3 has a very poor crystal quality, and moreover it can be obtained only in the form of a large crystal having an etch pit density higher than 10.sup.5 pieces/cm.sup.2. This makes it difficult to form an epitaxial film having excellent properties on the surface of a substrate having a poor crystal quality. It should be added that it is practically impossible to obtain a substrate having large dimensions.
A single crystal of LaGaO has a lattice constant a of 5.496 angstroms and a lattice constant b of 5.5554 angstroms. Thus, it is expected that the single crystal of LaGaO.sub.3 has an excellent lattice matching relative to the super-conductive material composed of an oxide. However, since phase transition takes place at a temperature of about 150.degree. C., there arises a problem that a twin crystal is contained in the single crystal. For this reason, removal of the twin crystal becomes .a significant subject to be solved when a substrate for forming a super-conductive thin film composed of a single crystal of LaGaO.sub.3 is used practically.
In addition, since the single crystal of LaAlO.sub.3 has lattice constants a and b of 3.788 angstroms, it is expected that it has an excellent lattice matching relative to the super-conductive material composed of an oxide. With respect to the single crystal of LaAlO.sub.3, however, it is very difficult to practically form a single crystal because of a very high melting temperature of 2100.degree. C. Another problem is that a twin crystal is contained in the single crystal.
With respect to the conventional materials which have been heretofore used as a substrate for forming a super-conductive thin film in the above-described manner, there arise problems that each of the conventional materials does not have an excellent lattice matching relative to the super-conductive thin film, it does not satisfactorily meet the requirement for easily purchasing a single crystal on the commercial basis and thereby it is substantially impossible to produce a stable super-conductive device.
The present invention has been made in consideration of the foregoing background. Therefore, an object of the present invention is to provide a material employable for a substrate of a single crystal which makes it possible to form an excellent epitaxial super-conductive thin film. Another object of the present invention is to provide a super-conductive thin film having a high quality which can be employed for a super-conductive device without unstable superconductivity inherent to a super-conductive thin film composed of an oxide.